Airbag devices in which an airbag for restraining knees of a passenger is provided so as to be connected with an airbag for restraining an upper body of the passenger are known (see, for example, Japanese Utility Model Application Laid-Open No. 7-17662 and Japanese Patent Application Laid-Open (JP-A) No. 51-5728). Further, there are known airbag devices that simultaneously inflate and expand, by a gas supply from a common pressure accumulator, an airbag for restraining an upper body of a passenger and an airbag for restraining knees of the passenger (see, for example, JP-A No. 51-23917). In the latter device, a check valve is provided respectively between the pressure accumulator and each airbag, and the internal pressure of each airbag is held at a predetermined value. Moreover, a technique is known of controlling the internal pressure of an airbag by the opening/closing of a vent hole (see, for example, JP-A No. 2004-262432 and JP-A No. 11-222097). Still further, there is known a technique in which a tube-shaped frame forming portion is provided along the inner surface of an airbag, and the airbag is inflated and expanded on the whole due to gas being supplied to the frame forming portion (see, for example, JP-A No. 2007-106376).